When the number of subscribers grows in the EPC (Evolved Packet Core) network every optimization possible is needed to fulfil the requirements from the industry.
The suggested way (see for instance 3GPP TS 23.401) to move a user equipment (UE) from EPS Connection Management ECM-IDLE state to ECM-Connected, meaning that the UE can receive downlink payload, is to do this on a UE level, this will make it impossible to initiate different paging procedures on a service level.
From 23.401, v8.5.0 the overall procedure looks like illustrated in FIG. 2.
If a mobility management entity (MME) needs to signal with the UE that it is in ECM-IDLE state, e.g. to perform an MME/HSS-initiated detach procedure for the ECM-IDLE mode UE or a Serving Gateway (SGW) receives control signalling (e.g. Create Dedicated Bearer Request or Modify Dedicated Bearer Request), the MME starts network triggered service request procedure from step 206.
If Idle mode Signalling Reduction (ISR) is activated, when the SGW receives a Create Dedicated Bearer Request or Modify Bearer Request for a UE, and the SGW does not have a downlink S1-U and the SGSN has notified the SGW that the UE has moved to PMM-IDLE or STANDBY state, the SGW buffers signalling messages and triggers MME and SGSN to page UE. In this case the SGW will be notified about the current Radio Access Technology (RAT) type based on the UE triggered service request procedure. The SGW will go on executing the dedicated bearer activation or dedicated bearer modification procedure, i.e. send the corresponding buffered signalling to MME or SGSN which UE resides in now and inform the current RAT type to a Packet Data Network Gateway (PGW) if the RAT type has been changed compared to the last reported RAT Type. If dynamic Policy and Charging Control (PCC) is deployed, the current RAT type information shall also be conveyed from the PGW to a Policy and Charging Rules Function (PCRF). If the PCRF response leads to an EPS bearer modification the PGW should initiate a bearer update procedure.
There are a number of problems with existing solutions, such as it is not possible to initiate different paging procedures for one specific UE or that there will be increased processing load in the MME since the MME has to page rather large areas to be able to secure that the UE is reached.